The present invention relates to an improvement of a driving system for moving paths or escalators which use an endless chain adapted to be circulated between a path for transporting passengers.
The moving path or escalator usually employed an endless chain applied between two sprockets disposed in a spaced relationship, one of the two sprockets is driven by an induction motor coupled therewith. In a horizontally movable moving path or an ascending escalator an induction motor is coupled to a sprocket installed at the destination side whereas in a descending escalator the induction motor is coupled to the sprocket installed at the entrance side. The endless chain for use in a long distance moving path or long stroke escalator is required to be of a high strength structure, because the chain is driven at one driving point and the whole tensile force concentrates at this driving point, resulting in an extremely magnified size of the whole system. In other words, as the passenger storing capacity of the system becomes large, a tensile force acting on the driving point of the endless chain increases. On the other hand, since the strength of the endless chain increases stepwise with a certain step, the endless chain requires a very great capacity as it becomes longer. The endless chain resistive to a large tensile force suffers from an increase in chain weight and in chain size. This magnifies stepwise the size of a driving device for driving the endless chain together with the loaded weight.
To eliminate these disadvantages, it has been suggested that another sprocket be provided at substantially the middle portion of the endless chain to suppress a maximum tension which the endless chain assumes and the sprocket is driven by a second induction motor for the purpose of smoothing the tension distribution in the endless chain.
In this prior art, however, a pair of induction motors are always in operation, which is disadvantageous in economical standpoint of view. Further, when both of the two induction motors are controlled, the control devices becomes complicated and expensive and reliability of the system is lowered. Further, there is an inconvenience that, when the system is loaded with a concentrated load at the intermediate portion between the two driving source sprockets, the second sprocket pushes the chain from the rear of the load with a result that there generates a minus tension in the chain between the first and second sprockets to provide a slackness of the chain so that the chain is off its guide.